Potential to Kinetic Energy
Principles of Energy Transfer
Objective: The objective of this demonstration is to investigate the principles of energy transfer.
Time: 25-30 minutes
Review of Scientific Principles
The law of conservation of energy states that energy can neither be created nor destroyed but can be changed into different forms. Potential energy is stored energy, and kinetic energy is energy of motion.
We will investigate the transfer of potential energy (PE) to kinetic energy (KE) with the use of a firecracker and soda cans. The firecracker has chemical potential energy that is released when it is ignited. This energy is changed to kinetic energy after the firecracker explodes, causing the cans to move. This process is similar to what happens when your car moves. The gas (PE) explodes in the cylinder (KE) throwing the piston down. This mechanical energy is eventually translated to movement of the wheels.
Gravitational Potential Energy= mass of an object (kg) x gravity (9.8m/s2) x height (m)
Unit conversions: 1 joule = 1 Nm = 1 kg m2/s2
Kinetic Energy = 1/2 x mass of an object (kg) x velocity 2 (m/s)2
Hypothesize: If the can were initially placed above the ground, what direction(s) would we see the cans traveling?
Materials and Supplies
- 2 empty soda cans
- a small firecracker
- meter stick
General Safety Guidelines
- When working with matches, be sure that you do not have bulky clothes which could catch on fire.
- Be sure that the area around the demonstration setup is clear, so that when you light the firecracker you have plenty of space to move away from it.
- Stay far enough away from the setup once you have ignited the firecracker (3 m).
- Set up your materials to look like the picture below.
- The height of the table should be one to two meters above the ground, making sure that it is level. Be sure to have at least 10 m radius clear around your table.
- Mass the two identical empty soda cans. Mass _____g
- After placing the cans end to end on the table, measure the distance from the bottom of the can to the floor. ________m
- Aim the cans such that they will not hit anyone or anything during their flight.
- Carefully fit an unlit small firecracker between the two cans.
- Mark the edge of the can towards the firecracker on the table with a pencil.
- Ignite the firecracker, and STEP BACK FROM SETUP AT LEAST 3 M.
- Measure the horizontal distance each can traveled from spot on the floor just below the original mark. __________ m ; __________ m
- Add the two distances and divide by two. The average distance is __________ m .
Calculations: (show your work)
- Falling and flying time of can Dv = 1/2gt2
- Horizontal velocity of can Dh = V t
- Kinetic energy of each can: KE = 1/2mv2
- Gravitational potential energy of each can: PE = mgDv
- Compare the PE and KE, are they equal?
- Should PE and KE be equal?
- Support your answer for # 2 with facts from this experiment.
- List the pathway the energy transformed into, from the firecracker being lit to the cans resting on the floor.
For Further Thought and Discussion:
- What do you think the sound of the firecracker exploding and the can hitting the floor might have to do with the released energy?
- What do you think the light or sparks from the fuse might have to do with energy? If there were no sparks, would the net energy released be greater or the same?
- In terms of molecular motion, where does some of the energy go when you shoot off a firecracker or bottle rocket?
- No see answer to #3.
- The kinetic energy of calculation 3 is from the firecracker. The potential energy of calculation 4 is from gravity and the height of the cans which has nothing to do with the firecracker.
- The match allows the chemical energy of the wick to transfer into light, sound, and heat energy. This heat energy set off the chemical reaction of the explosive which is converted to different forms of energy. This energy goes into light, sound, heat, and kinetic energy. This kinetic energy is absorbed by the cans which have also absorbed some of the heat of the explosion. As the cans move off of the support, their potential energy is converted into kinetic energy in the vertical direction. The cans continue to gain kinetic energy in the vertical direction until the cans hit the ground. At this point, the kinetic energy of the cans is converted into heat and noise of which the ground and air absorb.
For further thought and discussion:
- Sound is the transfer of energy in waves at audible frequencies. Some of the initial energy in the spark of the firecracker was lost to sound as the can hit the floor.
- Light is similar to sound in that it travels as both a wave and a particle. It takes energy to produce light waves. Some of the chemical energy went into making the light in the spark.
- When you shoot off a bottle rocket, the same principles of the experiment apply. The energy derived from the match is used to explode the firecracker. That then heats the molecules and creates an explosion forcing the bottle rocket up in the air. The remaining energy heats the molecules in and around the bottle itself, pushing it backwards.